This invention relates to the field of elevator control. In particular, the invention is intended for use with hydraulically-propelled elevators, i.e. elevators having cars which are pushed upward by extended pistons, the pistons being mounted for reciprocation within cylinders filled with hydraulic fluid. The main purpose of the invention is to insure that the elevator car will accelerate and decelerate uniformly, and over a fixed, predetermined time interval, regardless of the weight in the car.
It has long been known, in the field of hydraulically-driven elevator systems, that hydraulic pressure must be applied and released gradually, to avoid unpleasant jerks in the elevator ride. This problem is excerbated by the fact that the elevator industry generally uses synchronous motors, which stop and start immediately, when power is applied and cut off. Therefore, it has been known, in the prior art, to provide a control valve which cushions the flow of hydraulic fluid entering or leaving the main cylinder.
The control valves of the prior art are adjusted manually, by adjusting the sizes of their orifices, and by adjusting the stop positions of their moving elements. There may be as many as thirteen separate adjustments which can be made on one such valve. But after these valves have been adjusted, their settings are fixed. The valves are not designed to change their settings while the elevator is moving.
A valve setting which provides a smooth acceleration and deceleration for an empty car will not, in general be the optimum setting for a fully-loaded car. A change in load is not the only possible cause of sub-optimal performance. Changes in temperature affect the viscosity of the oil used as the hydraulic medium, and the fixed settings of the valves of the prior art will not compensate for these changes. The only way to adjust the elevator control valves of the prior art is to deactivate the system, gain access to the control valve, and adjust the orifice sizes as desired. Clearly, it is impractical to adjust the valve whenever the load on the elevator changes.
Valves which depend on adjustment of small orifices are also subject to clogging, due to particles of dirt in the hydraulic fluid. These particles can eventually change the effective orifice size, thus causing the effective settings of the valve to change in an unintended and unpredictable way.
The present invention solves the problems described above, by providing a "smart" valve which automatically adjusts itself according to the load in the elevator car, and according to the measured flow rate of hydraulic fluid through the valve.
The valves used in the prior art also generally require several solenoids, sometimes as many as five. One accomplishment of the present invention is the reduction of the number of solenoids to one.
The valve of the present invention is also much simpler, in construction and operation, than the control valves of the prior art. The valve of the present invention requires only a few electronic settings, which need not be changed during the entire life of the valve. It does not employ small orifices which could cause clogging. The invention provides a feedback loop, which allows the valve to adjust itself in response to changing conditions.